1. Field of the Invention
The invention relates to a TFT, and more particularly to a TFT with a self-aligned LDD and a method for fabricating the same.
2. Description of the Related Art
Thin-film transistors (TFTs) are widely used in active matrix liquid crystal display (AMLCD). A leakage current occurs, however, when the TFT is turned off, and the charges in the active matrix liquid crystal display are thereby lost. A lightly doped drain (LDD) structure has been used to reduce the electric field of the surface of the drain, such that the leakage current can be reduced.
In FIGS. 1A to 1B are cross-sections of the conventional method for fabricating a TFT with a self-aligned LDD.
In FIG. 1A, a transparent insulating substrate 101 is provided, a semiconductor layer 102 and a gate insulating layer 104 formed on the substrate 101, and the semiconductor layer 102 is covered by the gate insulating layer 104.
A patterned photoresist layer 106 is formed on the gate insulating layer 104. Heavy doping is performed on the semiconductor layer 102 to form a heavily doped region 108 as a source/drain (S/D) region except in the region covered by the photoresist layer 106.
In FIG. 1B, the photoresist layer 106 is removed. A gate layer 120 (e.g., a conductive layer) is formed on the gate insulating layer 104, and the gate layer 120 covers a portion of the undoped region of the semiconductor layer 102. Light doping is performed on the undoped region of the semiconductor layer 102 to form a LDD region using the gate layer 120 as a mask. The region of the semiconductor layer 102a that is covered by the gate layer 120 is a channel.
The pattern in the photoresist layer 106 defined by an additional mask in the conventional method results in a process that requires a separate masking step and doping procedure for forming the LDD region, which region is easily shifted by misalignment of the mask such that the electrical properties of TFT are affected.